I used to keep it in folders and subfolders but year and then location but since quite some years ago I shifted to just keep them on iCloud to access them on Mac or on the phone. And then I favourite the best ones and I save the other ones in an album called observations. I always fill the metadata if missing (scuba diving, I.e) and use it to view the photos. The search feature became quite good with many animals general categories but I also save its binomial nomenclature in description.
But I have to say, since I use iNaturalist I upload there the anatomical pictures and delete them from my device and cloud leaving only the “pretty ones”. Also, since I use iNaturalist I take more shots from different angles than are only for anatomical descriptions… specially with invertebrates where I end up with many macro from many angles of the same bug.
I love that since my photos are almost half nature and mostly critters, the apple algorithm just embraces that and regularly suggest automatic memories of “Birds over the year”, “underwater summer 2024” and I just dive in briefly in those pictures… and I just love it! 
I feel I have a better use of them this way instead of just collecting them in folders and always checking the same ones. Which is also why I keep a austerity policy with less pictures is better and tend to delete to keep only good one and leave the different shots of the same one in iNaturalist for when I want to focus on that aspect.